1. Field of the Invention
The invention relates in general to systems for guiding a light or infrared beam by means of a refraction index gradient produced by a temperature gradient in a gas.
More specifically, the invention relates to a guide for light or infrared waves mainly comprising a sectional metal tube twisted in a helix having a very long pitch relative to its cross-section, the center of the guide, where the beam propagates, containing air which flows slowly and without turbulence, a suitable temperature gradient being maintained in the guide by electric wires heated by Joule effect and borne by an insulating frame secured to the wall.
2. Description of the Prior Art
U.S. Pat. No. 3,415,588 issued Dec. 10, 1968 to Dwight W. Berreman has proposed to introduce moderate temperature gradients transversely to the path traversed by a ultrahigh frequency wave energy beam in an enclosing conduit, the gradient being distributed along the path and varied in their transverse angular directions in such manner that rays which tend to diverge from the axis of the beam are repeatedly deflected toward the axis in such the same way as they would be were a plurality of thin, long focal length, solid leanses introduced at appropriate intervals along the axis of the beam. The resultant collimating effect can then be proportioned to substantially eliminate the deleterious effects otherwise introduced by the tendency of the beam to spread.
The temperature gradient and thereby a refractive index gradient is produced and maintained by heating means longitudinally distributed within an elongated gas-filled waveguide to periodically encircle a portion of the space enclosed by the waveguide. The heating means can include a pair of interwined helical members extending along the waveguide or a plurality of discrete ring members spaced at intervals along the conduit. In both cases, the isothermic curves are complicated curves and the wall of the enclosing conduit or waveguide does not coincide with an insothermic surface.
In the prior art, articles by the present Applicant entitled "Guidance of coherent light by a helical guide" in "Annales des Telecommunications", May-June 1969, No. 5-6, Volume 24, pages 177-189 and September-October 1970, Bo. 9-10, Volume 25, pages 320-324, disclose a guide for light wave beams made up of four metal tubes disposed in a helix aound the beam propagation axis. Referring by anticipation to FIG. 1, which represents an aforementioned guide, it can be seen that the cross-sections of the four tubes 1-4 are centered at the four corners of a square and are at a tangent to a single circle centered at the beam axis and having a diameter equal to that of the tubes. A stream of warm water flows through two of the diametrically opposite tubes 1-3 whereas a stream of cold water flows through the other tubes 2-4, so that the isothermic curves in the air between the tubes are equilateral hyperbolas. The circular cross-section of the walls of the four tubes are identical with the osculatory circles at the limiting hyperbolas.
When the cross-section assembly shown in FIG. 1 moves along the beam axis, it rotates through an angle proportional to the distance travelled. The aforementioned articles contain a mathematical description of the propagation of light waves in a system of the aforementioned kind, and the possible guide curvature is calculated. It has also been shown by the present Applicant that the possibilities of curvature can be increased by curving the guide axis not in a circle but in a helix which winds around a circle.
Since the diameter of the free space in which the beam propagates is of the order of a centimeter and the guide is bent in a helix having a pitch of several meters, the position of the isotherms in the guide can with very close approximation be calculated as if the system were cylindrical.
The prior-art system was constructed from standard copper tubes used in plumbing; it showed the correctness of the propagation calculations in the aforementioned article. It was thus possible to show that, to obtain stable isotherms, the guide must be heat-insulated, in which case the cross-section is as shown in FIG. 2, which shows the four tubes 1-4, the sheath 5 and isotherms 6 drawn in thin lines.